more info about the hreflang attribute can be found at http://www.w3schools.com/tags/att_hreflang.asp Many websites serve users from around the world with content translated or targeted to users in a certain region. Google uses the rel="alternate" hreflang="x" attributes to serve the correct language or regional URL in Search results. more info can be found at https://support.google.com/webmasters/answer/189077?hl=en
It is a HTML tag that goes within the head section of the page, which helps Google crawlers understand that certain pages, section, sub domain or country code level domains is targeted for a specified country.